1. Field of the Invention
The present invention relates to a structure of an image sensor. More particularly, the present invention relates to a structure of a complementary metal oxide semiconductor (CMOS) image sensor.
2. Description of the Related Art
Most of the conventional digital sensors for image capture are charge-coupled devices (CCDs). CCD applications include monitors, video recorders, cameras, etc. However, CCDs have disadvantages such as high fabrication costs and limitations in size reduction. Thus, due to the current tendency to reduce costs and dimensions, a CMOS photodiode image sensor has been developed. Because the CMOS photodiode image sensor can be produced by conventional techniques and its costs and size can be reduced, it is predicted that CMOS photodiode image sensors will play a main role in sensors of the future.
A CMOS photodiode image sensor is substantially divided into a photo-sensing region, which is constructed by photodiodes, and a circuit region. A photodiode is a photo-sensitive device, also known as a photo-detecting device. The photodiode is based on the theory that a P-N junction can convert light into an electrical signal. In the absence of light, because the electrical field exists at a P-N junction, electrons in an N-doped layer and holes in a P-doped layer do not diffuse across the P-N junction, whereas when enough light strikes a photodiode of the photo-sensing region, electron-hole pairs are created and diffuse toward the P-N junction. As a result of an electrical field effect, the electrons flow to the N-doped region and the holes flow to the P-doped region. Thus, current is induced between P-N junction electrodes. Ideally, a photodiode in the dark is an open-circuit. In other words, no current is induced while the photodiode is in the dark.
In a conventional CMOS sensor, an N-type doped region is formed in a surface portion of a P-well. A depletion region formed at the junction between a P-well and the N-type doped region serves as a P-N junction. Since red light, blue light, and green light have different wavelengths, a CMOS sensor with a full color absorbency cannot be achieved by simply using the depletion region at the junction between the P-well and the N-type doped region.
The invention provides an image sensor structure. A substrate of a first conductive type is provided. A well of a second conductive type is located in the substrate. At least a first doped region of the first conductive type is located in a surface portion of the well. At least a second doped region of the first conductive type is located in a surface portion of the well. The first doped region is deeper than the second doped region. The first doped region and the second doped region are alternating, concentric regions. A terminal is located in the well. The first terminal connects to a source/drain region of a transistor of a circuit region. A contact is coupled with the well. The contact connects to a gate of a second transistor.
The invention also provides an image sensor structure. A first PN photodiode is located in a photo-sensing region of a substrate. A second PN photodiode is located in the substrate above the first PN photodiode. An N-type terminal of the first PN photodiode connects to a source/drain region of a first transistor. A contact is coupled with the second PN photodiode. The contact connects to a gate of a second transistor.
The present invention forms a photo-sensing region, which is constructed by a double photodiode. The double photodiode comprises the second photodiode above the first photodiode. Thus, red light, green light, and blue light are effectively absorbed. The image sensor having a full color absorbency is formed. In addition, the image sensor is formed without increasing the layout area compared with the conventional structure.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.